In a receiving side of a high speed data transmission system, when distinguishing transmitted data from received symbols disturbed by Inter-symbol Interference (hereinafter referred to as ISI) and Additive White Gaussian Noise (hereinafter referred to as AWGN), energy of the received symbols can be utilized to the greatest effect in signal determination by using Maximum Likelihood Sequence Estimation (MLSE) so that a best error rate can be theoretically realized. In maximum likelihood sequence estimation, it is known that a Viterbi decoder enables efficient circuit implementation. However, if ISI occurs over many symbols, the number of states of the Viterbi decoder becomes very large and circuit implementation cannot be feasible with realistic complexity.
For example, in case of transmission of binary data symbols is transmitted via a transmission path in which ISI occurs across a range of 10 symbols, the number of states of the Viterbi decoder becomes 1024, and implementation of a circuit that operates in a GHz band is impossible.
As a technique for reducing the number of states of the Viterbi decoder, a series of technologies referred to as Reduced State Sequence Estimation (RSSE) is known.
In particular, in reducing the number of states of the Viterbi decoder when the ISI occurs across many symbols, a delayed decision feedback sequence estimator (hereinafter referred to as DDFSE) which reduces the number of states of the Viterbi decoder by combining the Viterbi decoder and a decision feedback equalizer, is effective. Regarding theoretical details of the DDFSE, reference may be done to Non-Patent Document 1 (“Delayed Decision-Feedback Sequence Estimation”, by Alexandra Duel-Hallen and Chris Heegard, 1989, IEEE Transactions on Communications).
FIG. 20 is a diagram illustrating the impulse response of a transmission line distortion disclosed in Patent Document 1. In FIG. 20, a0 and a1 are precursor and center components estimated by Viterbi algorithm, respectively, and a2 and a3 are post cursor components which are removed by using a signal estimated by a0 and a1. When binary data {1, −1} is transmitted via a transmission line of a type that has an impulse response formed of one precursor ISI tap, a main tap, and a plurality of postcursor ISI taps, the received data is greatly disturbed by the ISI. For this received data, when maximum likelihood sequence estimation is performed using a Viterbi algorithm, processing of a trellis diagram with a number of states of 2 (number of precursor ISI taps+postcursor ISI tap number) is necessary.
FIG. 21 is a diagram showing the configuration of the conventional delayed decision feedback sequence estimator disclosed in Patent Document 1. FIG. 22 is a state transition diagram in the Viterbi algorithm disclosed in Patent Document 1. The effect of the postcursor ISI is removed from received data which is disturbed by ISI, by first and second DFEs 213 and 214. The number of states of the Viterbi algorithm, as shown in the trellis diagrams in FIG. 22, is reduced to 2, and the maximum likelihood sequence estimation is efficiently performed based on this reduced trellis diagram.
The reason that 2 DFEs 213 and 214 are necessary in FIG. 21 is that the number of states of the trellis after reduction is 2, and a DFE is necessary for each state. Output of a first provisional decision unit 220 of FIG. 21 is supplied to the first DFE 213, and is used in processing received data at a subsequent point in time.
In Patent Document 1, as shown in FIG. 21, the first DFE 213, a subtractor 24, a squaring calculator 25, an adder 26, a first compare-select circuit 29, and a first provisional decision unit 211 form a feedback loop, and determine an upper bound of processing speed. Therefore, in Patent Document 1, by performing tap calculation in advance for the DFE corresponding to a first postcursor ISI tap, processing time for the feedback loop is shortened. FIG. 23 is a diagram shown disclosed in Patent Document 1. The feedback loop is configured from a first selector 118, a subtractor 15, a squaring calculator 16, an adder 17a, and a first compare-select circuit 121. Shortening of the feedback loop of FIG. 23 is achieved as compared with the feedback loop of FIG. 21.
One of pipelined DFE computation is disclosed in Non-Patent Document 2 (Erich F. Haratesh, “New architectures for reduced-state sequence detection with local feedback”, International Symposium on VLSI Design, Automation and Test).    [Patent Document 1] JP Patent Kokai Publication No. JP-A-10-22879    [Non-Patent Document 1] Alexandra Duel-Hallen and Chris Heegard, “Delayed Decision-Feedback Sequence Estimation”, IEEE Transactions on Communications, 1989    [Non-Patent Document 2] Erich F. Haratesh, “New architectures for reduced-state sequence detection with local feedback”, International Symposium on VLSI Design, Automation and Test